Typically, electronic engine controls for use in vehicles include a cruise control which allows an operator to select and maintain a desired vehicle speed under usual operating conditions without the necessity of operating the throttle control. Such cruise controls, however, are not capable of maintaining speed regulation when the engine is operating outside predetermined limits. For example, the actual vehicle speed may drop below or rise above a permissible range of vehicle speeds surrounding the selected speed when the vehicle is ascending or descending a hill, when changes in altitude render it impossible to maintain the vehicle speed, when wind or other external factors provide sufficient forces on the vehicle to oppose or aid the power developed by the engine, etc. In such a case, it is necessary for the operator to manually intervene if it is desired to maintain the speed of the vehicle within the permissible range. In the case of speeds below the permissible range, it is typically necessary for the operator to downshift in order to provide greater torque to the wheels s that the vehicle speed can be increased. This manual downshifting is objectionable and should be eliminated, if possible.
Prior art approaches to the problem of downshifting while operating in cruise control have involved the use of a semi-automatic transmission which automatically downshifts the transmission without leaving the cruise control mode when the vehicle speed reaches a predetermined limit below the desired or commanded speed. Such an approach reduces the number of manual transmission downshifts, but does not reduce the total number of gear shifts. Thus, the demands placed upon the transmission are not lessened.
The following patents disclose engine controls but do not address the problems noted above while operating in a cruise control mode.
Thompson et al., U.S. Pat. No. 4,493,303 discloses an engine control wherein data representing a plurality of separate power curves are stored in a memory and the data are used to control fuel rack limits. The engine is typically utilized with a transmission having different gear ranges. During operation of the engine, the particular gear range of the transmission is detected by sensing road speed and engine speed and the data representing a particular power curve are retrieved from the memory in dependence upon the detected gear range. The data are used to establish the rack limits during operation in such gear range in order to produce desired operating characteristics and fuel economy. This patent also discloses the use of a cruise control, although the problems noted above are not even recognized.
Stevenson et al., U.S. Pat. No. 4,368,705 discloses an engine control system in which a throttle controls the delivery of fuel by a fuel pump to the engine. The control system further includes a fuel pump rack limit circuit which controls the rate of fuel delivery to the engine in accordance with rack limits and a timing circuit which controls the timing of injection of fuel into the engine cylinders. The timing and rack limits are established in accordance with engine conditions to obtain maximum engine performance with smoke and emission levels limited to those required by the Environmental Protection Agency (EPA).
More particularly, a rack limit position map is stored in a memory and includes rack limit position values which are predetermined in accordance with the physical characteristics of the fuel pump. This map is coupled to a first input of a least wins logic selector circuit which is in turn coupled to a rack limit control loop. A second input of the least wins logic selector circuit is coupled to the output of a summing junction which in turn sums the output of a torque rise limit map and an altitude derating map stored in additional memories. The torque rise limit map develops rack limit position control signal values which are predetermined to allow more fuel to be injected in the engine cylinders at lower speeds to prevent engine lugging without exceeding EPA emissions standards. The altitude derating map develops rack limit position control signal values which are predetermined to meet EPA standards for different levels of ambient air pressure. These values decrease the fuel rack limits at higher altitudes to maintain an optimal fuel/air ratio.
The least wins logic selector circuit selects the lesser of the output from the rack limit position map and the summing junction and provides such lesser value to the rack position control loop. Thus, the rack limit is set by the lesser of the rack limit position map and the torque rise limit map as derated by the altitude derating map output.
The rack limit is used to control the maximum rack position of the fuel pump so that the rate of fuel delivery may not be increased by the throttle beyond the rack limit.
Earleson et al., U.S. Pat. No. 4,498,016 discloses a governor control for an engine in which a desired power setting is used to develop a speed error and the speed error is in turn used to determine a desired rack position for a rack actuator that controls the fuel delivery rate to the engine. The desired rack position is compared with the actual rack position to create a rack position error signal, which is in turn used by a rack position control loop to drive the rack position toward the desired position. If a power setting is issued for positive acceleration, it is possible that the commanded acceleration of the engine will require more rack than is called for from the rack position actuator, thereby producing a negative rack error while a power increase is commanded. The resulting "dip" is eliminated by temporarily adding an acceleration signal to the position error signal to compensate for the lag in the desired rack position signal.
The present invention is directed to overcoming one or more of the problems set forth above.